First of all - make sure you have the most up-to-date patch; this was supposed to have been fixed a whlie ago.
To cure the existing zombies:
- if you have the Plants vs. Zombies content pack, you can put a Peashooter on your lot and "Have Peashooter Attack A Zombie"
- if you have an alchemist in the house, you could throw a Cure Elixir at each zombie.
- if you have a witch who knows the Sunlight Charm, that will also cure zombification.
resetsim * might fix all the zombies, but I'm not positive of that.